Field of the Invention
The present invention relates to a method for producing a dispersion liquid of a silica particle, and particularly to a method for producing a dispersion liquid of a silica particle useful for an abrasive used in polishing during formation of a metal wiring layer on a semiconductor integrated circuit, etc.
Description of the Related Art
Various integrated circuits are used for computers and various electronic devices. As they are downsized and become sophisticated, higher density and higher performance have been required for circuits.
To produce a semiconductor integrated circuit, for example, an interlayer film (insulating film) is formed on a substrate such as a silicon wafer, a groove pattern for a metal wiring is formed on the interlayer film (insulating film), a barrier metal layer of tantalum nitride (TaN) or the like is formed by a sputtering method or the like, as necessary, and then a copper film for a metal wiring is formed by a chemical vapor deposition (CVD) method or the like. Here, when a barrier metal layer of TaN or the like is provided, the barrier metal layer can prevent, for example, lowering of the insulation properties of the interlayer insulating film in association with the diffusion of copper or an impurity or the like to the interlayer insulating film and erosion of the interlayer insulating film, and can enhance the adhesion between the interlayer insulating film and copper.
Subsequently, a film of unnecessary copper and barrier metal (occasionally referred to as “sacrifice layer”) formed on a portion out of the groove is removed by polishing with a chemical-mechanical polishing (CMP) method and the upper surface is flattened to the maximum extent possible, and thus a wiring/circuit pattern of copper is formed by leaving a metal film only in the groove.
The abrasive used in the CMP method is typically produced by adding an oxidant for increasing the polishing rate of a wiring/circuit metal and an additive such as organic acid to a dispersion liquid comprising a spherical polishing particle consisting of a metal oxide such as silica and having an average particle diameter of about 5 to 300 nm.
When incompletely reacted materials (by-products) such as oligomers of silane alkoxide are present in the dispersion liquid comprising the polishing particle (silica sol), the silica sol has not had a sufficient stability, presumably due to the influence of the incompletely reacted materials such as highly reactive oligomers. Further, thickening, aggregation, white turbidity, generation of a sedimentary gel, etc. sometimes have occurred due to the influence of additives mixed to the dispersion liquid when used as an abrasive. Use of such an abrasive sometimes has resulted in appearance of scratches due to aggregates and has caused a problem by silica components remaining on a substrate after polishing (e.g., see Japanese Patent Laid-Open Publication Nos. 2015-124231, 2012-156393, and 2014-154707). Furthermore, in some cases, the silica sol has adsorbed an additive for enhancing the abrasive property, resulting in reduction in the effect of the additive.
As a method for producing a silica sol in which the generation of incompletely reacted materials such as such oligomers is suppressed, for example, proposed is a method which comprises (a) a step of adding an organic solvent containing tetramethoxysilane and a solvent containing an alkali catalyst and water to an organic solvent containing an alkali catalyst and water to cause hydrolysis and polycondensation of tetramethoxysilane, so as to produce a silica sol; and (b) a step of heating a dispersion medium of the silica sol to a boiling point of water to substitute with water (see Japanese Patent No. 4566645).